The present invention relates to a lens holding structure for modern compact cameras, wherein synthetic resin lenses are sometimes used in combination with glass lenses for their low weight and ease of manufacture.
However, synthetic resin differs markedly from glass in several mechanical properties, notably in hardness, mechanical stiffness, and coefficient of thermal expansion. If both materials are used in the same lens barrel, a lens holding structure must compensate for these differences so that there is no optical distortion.
There is known in the art a lens holding arrangement wherein a conical spring washer is disposed between a first lens and a second lens (U.S. Pat. No. 5,177,641), to allow for thermal expansion of the synthetic resin second lens. In the disclosed arrangement, the first lens is fixed and the second lens is movable in the direction of the optical axis. The conical spring washer provides a bias to separate the lenses, and the first and second lenses are thus maintained with a predetermined spacing. The conical spring washer acts to absorb shock in the direction of the optical axis.
However, if the described annular spring supports a plurality of lenses including at least one synthetic resin lens, then if the lenses in the group undergo shock, the deformation of the annular spring then increases because of the larger mass of a multi-lens group. The deformation of the spring may exceed the elastic limit of the spring. In a worst case situation, the resiliency of the annular spring may be weakened or lost when subjected to repeated shock.
Especially in a lens barrel, the width (in the radial direction) of the annular spring is constrained by the spatial limits of a lens barrel. The spring constant of the annular spring is constrained in the range where the spring will not cause distortion of the plastic lens under normal temperature variations. Given these limits, when the interval between the lenses is relatively large, the range of springs actually employable in a lens barrel is restricted to springs having a low elastic limit, and which may be permanently deformed under full compression.
Accordingly, when the interval between lenses is relatively large, the annular spring may be compressed in excess of its elastic limit and the lenses may be shifted by a large amount under shock load.